The present relates, in general, to magnetostrictive position transducers.
The phenomenon of magnetostriction has been widely employed in linear distance and position measuring devices. A magnet located near or around a magnetostrictive wire marks the location to be measured. Such devices can operate with either mechanical or electrical excitation. When an acoustical/mechanical strain propagating along the wire reaches the area of influence of the magnet, an electrical signal is generated in the wire. Conversely, when an electrical signal propagating along the wire reaches the area of influence of the magnet, a torsional strain is generated in the wire.
Such linear position detectors using a magnet mounted in a float have been utilized as liquid level detectors to provide an indication of a level of liquid within a tank, such as an underground tank. The position of the magnet, and hence, the liquid level, is determined as a function of the time required for a torsional disturbance to propagate from one end of the wire through the area of influence of the magnet, in the case of mechanical excitation, or from the position of the magnet to a sensing apparatus located at one end of the wire in the case of electrical excitation.
Other types of magnetostrictive position measuring devices utilize a reflective termination at the foot end of the magnetostrictive wire. Such devices measure the difference between the propagation times of a pulse from the magnet position to the foot of the wire and reflected back to the head of the device and a pulse traveling on the wire directly from the magnet to the head.
As shown in U.S. Pat. Nos. 4,839,590; 5,017,867; 5,050,430; and 5,253,521, all of which are assigned to the assignee of the present invention, such magnetostrictive devices include an elongated, small diameter tube, typically made of stainless steel, on which a movable magnet is mounted for providing an indication of a liquid level. An enlarged head and cap are mounted on one end of the tube, typically above the liquid level and enclose electronic components, such as terminal connections and a signal conditioning circuit used to supply or output signals to and from the magnetostrictive wire in the tube. As shown in these prior art patents, the head has a considerably larger diameter than the diameter of the elongated tube extending through the tank.
Fluid tanks, such as underground gasoline or fuel tanks, are typically provided with one or more risers or hollow conduits mounted on a top surface of the tank and extending upward to an upper end which is located at ground level. One riser is usually employed for filling the tank and typically will have a diameter of approximately four inches. A second smaller riser, such as a two inch diameter riser, may also be provided as a vent.
Existing tanks which are buried underground and which were not provided with the expectation of using liquid level detection within the tank, typically will not have a large riser or a riser dedicated to liquid level detection. Thus, where it is desirable to provide liquid level detection to such existing underground tanks, previously devised magnetostrictive positioning measuring devices having an enlarged head cannot fit within the fill nozzle of the existing riser on such tanks or will completely fill the interior of the riser thereby preventing the insertion of a fuel nozzle into the riser. The only recourse in such situations is to dig up the existing tank and add a riser specifically for a liquid level detection apparatus.
Thus, it would be desirable to provide a liquid level or position detection apparatus which can be easily mounted in tanks or vessels having small diameter risers. It would also be desirable to provide a liquid level or position detection apparatus which can be easily mounted in existing underground tanks without the need for digging up the tank. It would also be desirable to provide a liquid level detection apparatus in which all of the components of the apparatus are contained within a small diameter tube which is insertable into the tank. It would also be desirable to provide a position detection apparatus which can be mounted in any linear measurement device, such as a pressurized fluid operated cylinder for detecting the position of the piston of the cylinder in all mounting positions of the cylinder.
The present invention is a position detector suitable for detecting the position of a movable member carrying a magnet.
In one embodiment, the present invention is a position detector apparatus having a magnetostrictive wire extending through a measurement field with at least one magnet moveable through the measurement field for providing disturbances in the magnetostrictive wire. The apparatus comprises: an outer magnetic transmissive tube, having opposed first and second ends, a magnetostrictive wire is fixedly mounted within the outer tube. Pickup means are mounted within the outer tube and coupled to the magnetostrictive wire for outputting signals propagating along the magnetostrictive wire. Circuit means are disposed completely within the outer tube for coupling electrical signals to the magnetostrictive wire and the pickup means and for transmitting electric signals from the magnetostrictive wire and the pickup means. Fitting means are fixedly mounted in the first end of the outer housing, the fitting means containing a plurality of pins electrically engagable at one end to conductors extending from the circuit means and to external conductors at another end.
In another embodiment, the position detector is adapted for use with a fluid operated cylinder having a piston slidably mounted within a chamber between opposed first and second end walls and a piston rod fixed on the piston and extensibly projecting through an aperture in the second end wall. The position detector comprises an outer magnetic transmissive tube having opposed first and second ends. A magnetostrictive wire is fixedly mounted within the outer tube. Pickup means are mounted within the outer tube and coupled to the magnetostrictive wire for outputting signals propagating along the magnetostrictive wire. Circuit means are disposed completely within the outer tube for coupling electrical signals to the magnetostrictive wire and the pickup means and for transmitting electric signals from the magnetostrictive wire and the pickup means.
A magnet is mounted on the piston. The piston rod is hollow and slidably receiving the outer tube therein. Connector means are carried on a first end of the outer tube for fixedly mounting the first end of the outer tube to the first end wall of the cylinder the fitting means being disposed completely within the external surface of the first end wall.
The apparatus also defines an embodiment wherein the circuit board is fixedly mounted on the inner tube carrying the magnetostrictive wire. Means are provided for centering the circuit board within the outer tube. In a preferred example, the centering means comprises an annular disc mounted about the circuit board and having an outer annular groove. The outer tube is crimped at the position of the annular groove in the disc to fixedly join the outer tube to the disc and thereby fixedly position the circuit board and the inner tube relative to the outer tube.
The present invention provides several improvements over previously devised position detectors or liquid level detectors employing magnetostrictive waveguides. In addition, a unique flush mount adapter is provided for the position detector apparatus and mounted on one end of the outer tube for flush mounting within the end wall of a pressurized fluid operated cylinder. This enables such cylinders to use a position detector of the present invention in substantially all mounting positions of the cylinder, including positions where the end wall is fixedly joined to other components, such as a trunnion, etc.